fix: remove dead code (toast stubs, unused Redis key, tunnel manager fields)

- Remove 7 no-op exported stubs from toast.tsx (ToastProvider, ToastViewport,
  Toast, ToastTitle, ToastDescription, ToastClose, useToasts) — nothing imports them
- Remove fwFailKey variable and its Set() call from worker.go — the
  firmware:check-failed Redis key was never read anywhere
- Remove unused deviceStore and credCache fields from tunnel.Manager struct
  and drop corresponding parameters from NewManager(); update call site in
  main.go and all test usages

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/ui/toast.tsx

@@ -38,13 +38,3 @@ export function toast(options: ToastOptions) {
   }
 }
 
-// Backward-compatible no-op exports for AppLayout migration
-// These were used by the old Radix Toast implementation
-export const ToastProvider = ({ children }: { children: React.ReactNode }) => <>{children}</>
-export const ToastViewport = () => null
-export const Toast = () => null
-export const ToastTitle = () => null
-export const ToastDescription = () => null
-export const ToastClose = () => null
-// eslint-disable-next-line react-refresh/only-export-components
-export const useToasts = () => ({ toasts: [] as never[], dismiss: () => {} })